A radio frequency (hereinafter, RF) apparatus, such as an RF transceiver, may include various blocks of analog and digital circuitry. In digital RF transceivers, phase-locked loop (hereinafter, PLL) is used to demodulate and/or modulate the incoming and/or outgoing signals. Oscillators are used in electronic circuits and systems to provide a central clock signal, which is utilized to control the sequential operation of the entire system. A digitally controlled oscillator (hereinafter, DCO) is one of the most critical blocks in a digital PLL.
Different communication systems have different requirements of oscillation frequency. Thus, DCO design for cellular applications to achieve universal coverage for a wide range of frequencies in different communication standards is a trend. Conventionally, multiple DCOs are implemented to cover distinct bands required by different communication standards. However, hardware cost and power consumption of the DCO may increase.